a) Field of the Invention
The present invention relates to optical elements such as a vari-focal lens element, a vari-focal diffractive optical element and a variable declination prism which are to be used as liquid crystal optical elements. The present invention also relates to an electronic image pickup unit which uses these optical elements.
b) Description of the Prior Art
For composing a vari-focal lens system of lens elements which are manufactured by polishing a glass material, it is conventional to change a focal length of the lens system by moving a lens unit(s) in a direction along an optical axis, for example, as in a zoom lens system for cameras since the lens elements cannot change focal lengths by themselves. However, such a lens system has a drawback that it has a complicated mechanical structure.
For correcting such a drawback, there has been proposed an optical system which uses a polarizing plate 1 and a liquid crystal lens component 2, for example, as shown in FIG. 1. The liquid crystal lens component 2 comprises lens elements 3a and 3b, and a liquid crystal layer 5 which is disposed between these lens elements by way of transparent electrodes 4a and 4b, and an AC power source 7 is connected between the transparent electrodes 4a and 4b by way of a switch 6, whereby the optical system is configured to change a refractive index of the liquid crystal layer 5 by selectively applying an electric field to the liquid crystal layer 5.
When natural light, for example, is incident on the polarizing plate 1 of this optical system, only a predetermined linearly polarized component transmits through the polarizing plate 1 and is incident on the liquid crystal lens component 2. In a condition where the switch 6 is turned off and no electric field is applied to the liquid crystal layer 5 as shown in FIG. 1, longer axes of liquid crystal molecules 5a are oriented in a direction of a plane of polarization of the incident linearly polarized component, whereby a refractive index of the liquid crystal layer 5 is enhanced and a focal length of the liquid crystal lens component 2 is shortened. In a condition where the switch 6 is turned on and an electric field is applied to the liquid crystal layer 5 as shown in FIG. 2, in contrast, the longer axes of the liquid crystal molecules 5a are oriented in parallel with an optical axis, whereby the refractive index of the liquid crystal layer 5 is lowered and the focal length of the liquid crystal lens component 2 is prolonged. The focal length of the optical system shown in FIG. 1 is variable by selectively applying an electric field in the liquid crystal lens component 2 as described above.
However, the optical system shown in FIG. 1 poses a problem that it attenuates rays to be incident on the liquid crystal lens component 2 during transmission through the polarizing plate 1 and lowers a light utilization efficiency since it requires to dispose the polarizing plate 1 before the liquid crystal lens component 2 so that only the predetermined linearly polarized component is incident on the liquid crystal lens component 2. Further, the optical system which utilizes light at such a low efficiency poses another problem that it is applicable only to limited instruments or has a low versatility.
Further, an electronic image pickup unit for electronic cameras, video cameras and the like consists of a combination of an image pickup device 8 and a lens system 9 as shown in FIG. 3.
Such an electronic image pickup unit generally uses a lens system which has a relatively complicated composition, has a complicated configuration as a whole, comprises a large number of parts and requires tedious assembly, thereby being limited in compact design and reduction of a manufacturing cost thereof.